The number of molecules of water is calculated as follows
find the moles of water= moles=mass/molar mass
moles = 69.1g/18 g/mol = 3.84 moles
by use of Avogadro law constant
that is 1 mole = 6.02 x10^23 molecules what about 3.84 moles of water
the molecules of water = (3.84 x 6.02 x10^23) = 2.312 x10^24 molecules